The present invention relates to a high-strength steel sheet such as a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet or a high-strength surface-treated steel sheet, the formability after welding of which is excellent and, further, the fatigue property of which is excellent and, furthermore, the resistance to softening of the heat affected zone of which is excellent.
Conventionally, when bodies or parts for automobiles are produced, members of the automobiles are formed by means of press forming, and the thus formed members are integrated and assembled into one unit by means of spot welding or arc welding. Recently, in order to decrease the weight of an automobile body and enhance the yield of material for the object of reducing the production cost, they have made examinations into a method in which steel sheets, the mechanical strengths of which are different or the thicknesses of which are different, are integrated into one body by means of welding and then the thus integrated body is subjected to press forming. In order to reduce the weight of the automobile body, they have positively examined a method of applying high-strength steel sheets to the automobile body.
However, since the steel sheet has a weld zone and a heat affected zone, the following problems may be encountered in the process of press forming the sheet after welding. The formability of the steel sheet is deteriorated due to the cracks created in the process of press forming, which are not realized in the conventional manufacturing process in which welding is conducted after the process of press forming. Further, material in the heat affected zone is softened in the process of press forming, which is not realized in the conventional manufacturing process, either.
Improvements in the mechanical strength of the weld zone itself have been proposed by JP-A-3-199343, JP-A-5-186849 and others. However, in the techniques proposed by the above patent publications, forming is not conducted after welding. Therefore, the techniques proposed by the above patent publications are different from the technique in which press forming is conducted after welding. Concerning a method of enhancing the formability of a steel sheet after it has been welded, JP-A-7-26346 proposes such a method. According to this method, the components of ultra-low carbon steel are optimized so as to enhance the formability of a steel sheet after it has been welded. This method can realize excellent formability after welding compared with the formability of a conventional ultra-low carbon steel, however, the following problems may be encountered in this method.
The method of the above proposal is related to ultra-low carbon steel, the mechanical strength of which is relatively low. In order to further reduce the weight of an automobile body, it is necessary to apply the method to material of high mechanical strength. However, when the above method is applied to a high-strength steel sheet, the formability of the steel sheet after welding has not been clearly explained in a technical standpoint. Further, after the steel sheet has been welded, the mechanical strength of the steel sheet in the heat affected zone is deteriorated, that is, the heat affected zone is softened. For the above reasons, the reliability of the product is not high.
The parts used for an automobile are given a repeated load when the automobile is running. Therefore, it is desirable that the fatigue properties of both the base metal and the weld zone are excellent.
Concerning the fatigue property of a high-strength steel sheet, a large number of proposals have been made for a high-strength hot-rolled steel sheet, and a few proposals have been made for a high-strength cold-rolled steel sheet and a high-strength surface-treated steel sheet. In the above circumstances, there is proposed a technique, in JP-A-3-264646, in which the fatigue property can be improved when the steel structure is made to be a dual phase structure. However, according to the above technique, the press forming property after welding has not been clearly explained in a technical standpoint. That is, steel sheets in which the fatigue property and the press formability after welding are compatible with each other are not provided.
An object of the present invention is to solve the above problems. That is, the present invention has been accomplished to provide a high-strength steel sheet such as a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet or a high-strength surface-treated sheet, the press formability after welding of which is excellent and further the fatigue property of which is excellent, and furthermore the mechanical strength of the heat affected zone of which is not deteriorated.
In summary, the present invention to solve the above problems provides high-strength steel sheets described in the following items (1) to (8). Also, the present invention solves the above problems by providing high-strength steel sheets described in the following items (9) to (16).
(1) A high-strength steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A).
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(2) A high-strength steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A), and the dislocation density per plane visual field of 1 xcexcm2 is not less than 50/xcexcm2 and not more than 10,000/xcexcm2.
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(3) A high-strength steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Ti: 0.001 to 0.02%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A).
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(4) A high-strength steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Ti: 0.001 to 0.02%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A), and the dislocation density per plane visual field of 1 xcexcm2 is not less than 50/xcexcm2 and not more than 10,000/xcexcm2.
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(5) A high-strength steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, according to items (1), (2), (3) or (4), wherein the high-strength steel sheet is a high-strength hot-rolled steel sheet.
(6) A high-strength steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, according to items (1), (2), (3) or (4), wherein the high-strength steel sheet is a high-strength cold-rolled steel sheet.
(7) A high-strength steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, according to items (1), (2), (3), (4), (5) or (6), wherein the high-strength steel sheet is a high-strength surface-treated steel sheet.
(8) A high-strength surface-treated steel sheet having excellent formability and resistance to softening of the heat affected zone after welding, according to item (7), wherein the high-strength surface-treated steel sheet is a high-strength galvanized steel sheet.
(9) A high-strength steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Cu: 0.2 to 2.0%
Ni: 0.05 to 2.0%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A).
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(10) A high-strength steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Cu: 0.2 to 2.0%
Ni: 0.05 to 2.0%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A), and the dislocation density per plane visual field of 1 xcexcm2 is not less than 50/xcexcm2 and not more than 10,000/xcexcm2.
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(11) A high-strength steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Ti: 0.001 to 0.02%
Cu: 0.2 to 2.0%
Ni: 0.05 to 2.0%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A).
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(12) A high-strength steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, comprising the following components expressed by mass %,
C: 0.01 to 0.15%
Si: 0.005 to 1.0%
Mn: 0.1 to 2.2%
P: 0.001 to 0.06%
S: 0.001 to 0.01%
N: 0.0005 to 0.01%
Al: 0.001 to 0.1%
Nb: 0.005 to 0.05%
Mo: 0.05 to 0.5%
Ti: 0.001 to 0.02%
Cu: 0.2 to 2.0%
Ni: 0.05 to 2.0%
Fe: principal component
xe2x80x83where the components satisfy the following expression (A), and the dislocation density per plane visual field of 1 xcexcm2 is not less than 50/xcexcm2 and not more than 10,000/xcexcm2.
0.22xe2x89xa7C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%)xe2x80x83xe2x80x83(A)
(13) A high-strength steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, according to items (9), (10), (11) or (12), wherein the high-strength steel sheet is a high-strength hot-rolled steel sheet.
(14) A high-strength steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, according to items (9), (10), (11) or (12), wherein the high-strength steel sheet is a high-strength cold-rolled steel sheet.
(15) A high-strength steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, according to items (9), (10), (11), (12), (13) or (14), wherein the high-strength steel sheet is a high-strength surface-treated steel sheet.
(16) A high-strength surface-treated steel sheet having excellent fatigue property, excellent formability and resistance to softening of the heat affected zone after welding, according to item (15), wherein the high-strength surface-treated steel sheet is a high-strength galvanized steel sheet.